Synthesis of a highly crystalline, covalently linked porous network

ABSTRACT

Porous networks are described linked by boronates. Also described are processes for producing the porous networks. The porous networks are formed by reacting a polyboronic acid with itself or with a polydiol, a polydiamine, or a polyamino alcohol. The resulting boronate linkage is covalently bonded. The characteristics and properties of the resulting porous material can be varied and altered by changing the reactants and by incorporating functional groups into the reactants. Of particular advantage, the porous materials can be formed at or near atmospheric pressure and at low temperature in the presence of one or more solvents.

RELATED APPLICATIONS

The present application is a divisional of and claims priority to U.S.patent application Ser. No. 13/562,697 titled “Synthesis of a HighlyCrystalline, Covalently Linked Porous Network” of Lavigne, et al. filedon Jul. 31, 2012 now U.S. Pat. No. 8,829,065; is a divisional of andclaims priority to U.S. patent application Ser. No. 12/279,006 ofLavigne, et al. filed on Feb. 9, 2009 (now U.S. Pat. No. 8,258,197),which is a nationalization of International Patent Application No.PCT/US07/04802 filed on Feb. 23, 2007; and claims priority to U.S.Provisional Patent Application No. 60/776,707, filed on Feb. 24, 2006,all of which are incorporated by reference herein.

FEDERALLY SPONSORED RESEARCH

The present invention was developed with funding from the NationalScience Foundation under grant number CHE 0415553 and the AmericanChemical Society-Petroleum Research Fund under grant number 41833-G4.

BACKGROUND

(1) Field of the Invention

The present invention relates to the field of synthesizing crystalline,covalently linked porous networks with high surface area.

(2) Description of the Related Art

Porous materials are of great interest for applications in gas storage,catalysis and separations. Therefore, the ability to generate highlyporous, robust materials in an efficient and simple manner is greatlydesired. Metal-organic coordination networks have been vigorouslyinvestigated as a means to generate microporous frameworks. While therehave been numerous successes using this approach, it would be beneficialto have a means to assemble covalently linked networks for thegeneration of more structurally stable assemblies. Highly ordered,porous networks based on boronic acid building blocks have beenpreviously disclosed. However, these materials assemble through hydrogenbonding to form stable, highly interpenetrating, diamondoid frameworks.Further improvements in forming porous networks from boronic acids isstill needed. In particular, a need exists for a porous network that isbased on more stable covalent bonding. Also needed is an improvedprocess for producing the porous networks that does not have high energyrequirements.

SUMMARY

In general, the present disclosure is directed to an improved method ofsynthesizing and characterizing highly crystalline, covalently linked,porous organic networks under mild conditions. In one embodiment, forinstance, the porous network is formed from a boronate linked network.According to the present disclosure, for instance, porous networks canbe formed from dioxaborole, diazaborole, oxazaborole, or boroxine (oranhydride) linked networks.

The porous networks formed in accordance with the present disclosure canbe used in numerous applications. For instance, the porous product canbe used to reversibly absorb gases or other chemical species, such asorganic compounds. The porous networks can also be used as a catalyst orto support a catalyst during any suitable chemical reaction.

In one embodiment, for instance, a boronate porous network is formedaccording to the present disclosure by reacting and combining together afirst reactant with a second reactant. The first reactant comprises apolyboronic acid or an acyclic boronate ester thereof. For instance, thefirst reactant may comprise a diboronic acid, a triboronic acid, atetraboronic acid, or the like. The second reactant, on the other hand,may comprise a polydiol, a polyamino alcohol, or a polydiamine. As usedherein, the prefix “poly” means two or more. The first reactant isreacted with the second reactant to form a covalently bonded polymericor oligomeric porous network comprising a boronate linked network.

Of particular advantage, the present inventors have discovered that, inone embodiment, the above reaction can occur at or near atmosphericpressure and at a temperature of less than about 110° C. As used herein,at or near atmospheric pressure depends upon various factors includingthe particular reactants used and the temperature at which the reactiontakes place. At or near atmospheric pressure, however, at least is fromabout 0.75 atmospheres to about 1.25 atmospheres.

Thus, the porous networks of the present disclosure can be formedeconomically and under moderate conditions. The temperature at which theporous network is formed, for instance, can be from about 15° C. toabout 100° C., such as from about 20° C. to about 90° C. Duringformation of the porous network, the second reactant as described abovemay be added in excess in order to prevent the first reactant fromreacting with itself if desired.

During formation of the porous network, at least one solvent may bepresent. For example, a first solvent may be present that has a boilingpoint of less than about 110° C. For instance, the first solvent maycomprise tetrahydrofuran. A second solvent may also be present that may,for instance, improve the solubility of the polyboronic acid. The secondsolvent may also aid enhancing the fidelity of the product network byallowing the growing network to repair errors that may occur duringsynthesis given the reversibility of the boronate linkages. The secondsolvent may comprise, for instance, a low molecular weight alcohol. Forinstance, the second solvent may be methanol. The methanol may bepresent in an amount less than about 10% by volume, such as less thanabout 3% by volume.

As will be described in more detail below, various different polyboronicacids may be used to form the porous network. In addition, variousdifferent polydiols, various different polyamino alcohols, and variousdifferent polydiamines may also be used. In fact, the reactants can beselected so as to control the physical properties of the resultingproduct.

In one particular embodiment, a triboronic acid, such asbenzene-1,3,5-triboronic acid may be reacted with a bis-diol. Thebis-diol may comprise, for instance, 1,2,4,5-tetrahydroxybenzene.

The resulting product can be in the form of porous sheets stackedtogether. In one embodiment, the stacks of sheets may be arrangedtogether such that the pores between adjacent sheets are in alignment.In this manner, a porous network is formed having channels that extendthrough the stacked sheets. The diameter of the pores can vary dependingupon various factors. For instance, the pore diameter may be less thanabout 200 angstroms, such as less than about 70 angstroms, such as evenless than about 20 angstroms. Of particular advantage, the resultingpore structure can be thermally stable at temperatures of at least about500° C. The porous boronate linked network can have a surface area ofgreater than about 400 m²/g, such as greater than about 1200 m²/g. Thesurface area, however, can vary depending upon the particular reactantsthat are selected.

In an alternative embodiment, a porous network may be formed accordingto the present disclosure by reacting one or more polyboronic acidstogether. In this embodiment, a porous network is formed containingboroxine linkages. For instance, in one embodiment, a boroxine porousnetwork may be formed by reacting a triboronic acid with itself. Inparticular, a dehydration product is formed by reacting the boronic acidwith itself. Ultimately, an anhydride network is formed having arelatively small pore size.

Other features and aspects of the present disclosure are discussed ingreater detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure may be better understood with reference to thefollowing figures:

FIG. 1 is a schematic diagram showing one embodiment for a process forproducing a triboronic acid;

FIG. 2 is a schematic diagram for derivatizing triboronic acid forpurposes of characterization;

FIG. 3 are illustrations showing the crystal structure of the compoundproduced in FIG. 2;

FIG. 4 is a schematic diagram of one embodiment of a reaction forforming porous networks in accordance with the present disclosure;

FIGS. 5 through 14 depict the characterization results for the startingmaterials and products for the network produced in FIG. 4;

FIG. 15 is a schematic diagram of a tetraboronic acid and of oneembodiment of a reaction scheme for producing a porous network inaccordance with the present disclosure and gas adsorption datasignifying this is a porous network;

FIG. 16 is one embodiment of a reaction scheme for incorporatingfunctional groups into a polydiol that may be used in producing a porousnetwork in accordance with the present disclosure;

FIGS. 17 and 18 are chemical structures of porous networks that may beformed in accordance with the present disclosure; and

FIG. 19 is a schematic diagram of a porous network that may be made inaccordance with the present disclosure.

DETAILED DESCRIPTION

It is to be understood by one of ordinary skill in the art that thepresent discussion is a description of exemplary embodiments only, andis not intended as limiting the broader aspects of the presentinvention.

The generation of self-assembling materials based on the interactionsbetween poly-boronic acids and poly-diol compounds forming boronatelinked materials represents one method to generate stable, highlycrystalline porous covalent networks through a simple dehydrationprocess without the need for the addition of catalysts or otherreagents. Given the covalent yet reversible nature of the boronatelinkage, these assemblies form in a highly ordered manner. Compared toother covalently bonded organic counterparts, boronate-linked networksare assembled with greater ease and with higher efficiency; furthermore,compared to their non-covalently bonded equivalents, the boronate-linkedmaterials display enhanced stability.

Boronate linked porous networks have self-repairing capabilities and maycomprise self-assembling crystalline networks. Described below is anassembly motif for the porous material. Through characterization it hasbeen found that the boronate linked networks produce a highlycrystalline, covalent, microporous extended organic network that can besynthesized in good yields under moderate conditions. In one embodiment,the network may be assembled solely with boronate ester linkages.

In general, the porous networks of the present disclosure are formed byreacting a first reactant with a second reactant. The first reactant maycomprise, for instance, a polyboronic acid. The second reactant, on theother hand, may vary depending upon the particular application and thedesired results. The second reactant reacts with the first reactant toform a porous network, for example, linked through boronate esters. Inone embodiment, the second reactant may comprise a polydiol for forminga porous network linked through a dioxaborole. In an alternativeembodiment, the second reactant may comprise a polydiamine for producinga porous network linked through diazaboroles. In still anotherembodiment, the second reactant may comprise a polyamino alcohol forproducing a porous network linked through oxazaboroles. In still anotherembodiment, the second reactant may comprise the same or a differentpolyboronic acid for producing a porous network linked by boroxines (orboronanhydrides). When producing these boronate linked networks of thepresent disclosure, the polyboronic acid may first be converted to anacyclic boronate ester prior to reacting with the second reactant.

Through the above process, a covalently bonded polymeric or oligomericporous network containing boronate linkages is formed. It has beendiscovered that the covalent bonding interaction is also reversiblebetween the boronic acids and the second reactant. In addition, thereactants can be varied in order to produce a porous network havingdesired properties and characteristics.

The boronate linked networks made according to the present disclosureprovide various advantages and benefits. For instance, the boronatelinked networks are more stable than other self-assembling interactions,such as porous networks that rely on hydrogen bonding. Yet, theacids/ester equilibrium of the boronate linked networks is reversibleand retains all the benefits of a self-assembling system includingself-repair, efficient formation, and the like.

The boronate linked networks are also hydrolytically labile and arethermally stable. In particular, even though unhindered boronatesdegrade hydrolytically, sterically crowded boronates exhibit increasedstability. The boronate linked networks made according to the presentdisclosure, for instance, are thermally stable at temperatures of atleast 400° C., such as at least 500° C. In addition, the material can berepaired thermally when degraded. Also of advantage, changing the natureor valency of the building blocks used to form the boronate linkednetworks readily alters the assembled product outcome and properties ina predictable fashion. Thus, porous networks can be formed according tothe present disclosure that are tailored for a particular application.

In addition, as will be described in greater detail below, functionalgroups can be easily incorporated into the porous network. For instance,cyclic boronate esters quantitatively replace acyclic, mono-esters, thusother Lewis basic functionality can be incorporated into the estersduring formation. The functionality may comprise, for instance, amines,alcohols, thiols, and the like.

The boronate linked porous networks of the present disclosure can bemade using various techniques and methods. In one particular embodiment,for instance, the boronate linked networks may be formed through afacile dehydration process in one or more solvents. In particular, thepresent inventors have discovered that porous boronate linked networkscan be formed by simply warming the reactants in the presence of asolvent. For example, the compound selected to be reacted with thepolyboronic acid may, in one embodiment, be first combined with one ormore solvents. For example, the reactant, such as a polydiol, may befirst combined with a relatively low boiling point solvent in additionto a solvent that better solubilizes the polyboronic acid. Therelatively low boiling point solvent, for instance, may comprise anysuitable solvent that has a boiling point of less than about 100° C. Forinstance, in one embodiment, tetrahydrofuran may be used. Alternatively,water may be present.

The other solvent, on the other hand, may comprise any suitable alcohol.For instance, the alcohol may be a relatively low molecular weightalcohol, such as methanol, ethanol, a propanol or mixtures thereof. Thesolvents may be present during the reaction in an amount less than about10% by volume, such as in an amount from about 1% to about 3% by volume.

Next, a polyboronic acid may then be combined with the solution incontrolled amounts. In some embodiments, heat may be required for thereaction to proceed. Of particular advantage, however, the reaction canoccur at or near atmospheric pressure and at a temperature of less thanabout 100° C., such as from about 15° C. to about 90° C., such as fromabout 20° C. to about 70° C.

As described above, in one embodiment, the compound to be reacted withthe polyboronic acid may first be combined with a solvent prior to beingcontacted with the polyboronic acid. In other embodiments, however, itshould be understood that the reaction can also be done by dissolvingeach reactant independently and mixing these solutions or by mixing thetwo reactants together and then adding the one or more solvents.

In one embodiment, the reactant that reacts with the polyboronic acidmay be present in excess amounts so as to prevent the polyboronic acidfrom reacting with itself. As will be described below, however, whenproducing boroxines in accordance with the present disclosure, the firstand second reactants both comprise polyboronic acids.

If desired, the polyboronic acid may first be converted to an acyclicboronate ester prior to or during the reaction.

In one embodiment, the reaction can occur in an inert atmosphere so asto prevent any undesired reactions occurring with oxygen. The inertatmosphere, for instance, may comprise any atmosphere having low levelsof molecular oxygen. For instance, in one embodiment, the reaction canoccur under nitrogen.

It is believed that the reactants undergo a condensation reaction toform the covalently bonded boronate linked porous network. When reactinga polyboronic acid with a polydiol, the molar ratio between thepolyboronic acid and the polydiol may be from about 2:3 to about 1:2.5.

In general, any suitable polyboronic acid may be used to form porousnetworks in accordance with the present disclosure. In one particularembodiment, for instance, a triboronic acid may be used. In otherembodiments, however, a diboronic acid or a tetraboronic acid may alsobe used. A non-exhaustive list of examples of polyboronic acids that maybe used in the present disclosure include the following:

Boronic acids are also disclosed in U.S. Patent Application PublicationNo. 2006/0154807, which is incorporated herein by reference for allpurposes.

When using a tetraboronic acid, it is believed that a diamondoid networkmay be formed due to the 3-dimensional configuration of the boronicacid. For example, referring to FIG. 15, a tetraboronic acid is shownthat may be reacted with a bis-diol to produce a porous network. A gasadsorption profile for the resulting porous network is also illustratedin FIG. 15. In particular, the graph illustrates a molecular nitrogengas adsorption isotherm for the resulting network when using1,2,4,5-tetrahydroxybenzene. It is believed that this particular porousnetwork made according to the present disclosure is crystalline and hasa surface area of from about 400 m²/g to about 500 m²/g.

When using various other boronic acids, on the other hand, a more2-dimensional structure is formed in sheets. Each sheet includes pores.As the different sheets stack together, it is believed that the poresalign forming channels as will be described in greater detail below.

As described above, in one embodiment, the polyboronic acid may bereacted with a polydiol to form a porous network comprising dioxaborolelinkages. In general, any suitable polydiol may be combined and reactedwith the polyboronic acid. Examples of suitable polydiols include,without limitation, the following:

One particular reaction scheme for a polyboronic acid combined with apolydiol is illustrated in FIG. 4. In particular, in this embodiment, atriboronic acid, namely benzene-1,3,5-triboronic acid is reacted with abis-diol, namely 1,2,4,5-tetrahydroxybenzene. As shown in FIG. 4, acovalently bonded porous network is formed.

In an alternative embodiment, the following polyboronic acid may bereacted with the following polydiol to form the structure illustrated inFIG. 17.

In still another embodiment, the following polyboronic acid may becombined with the following polydiol to form the structure illustratedin FIG. 18.

As shown by comparing FIGS. 4, 17 and 18, various different porousnetworks having various different properties can be formed by alteringthe reactants.

In addition to reacting a polyboronic acid with a polydiol, in analternative embodiment, the polyboronic acid can be reacted with apolydiamine to form a porous network linked through diazaboroles.Examples of polydiamines that may be used include the following:

In still another embodiment, the polyboronic acid may be reacted with apolyamino alcohol in order to form a porous network linked viaoxazaboroles. One example, for instance, of a polyamino alcohol is asfollows:

Again, by changing the nature of the second reactant, a porous networkcan be formed with different properties and characteristics. Forinstance, it is believed that a porous network formed from a polyaminoalcohol may be more stable than a porous network formed from a polydiol.Porous networks linked via diazaboroles may in fact bend from planarity,raising the level of complexity in the resulting structure. Choosingbetween a polydiol, a polyamino alcohol, or a polydiamine may also causechanges in the affinity of the resulting porous network for particulargases and/or organic materials.

In still another embodiment of the present disclosure, a firstpolyboronic acid may be reacted with a second polyboronic acid to form aporous network linked via boroxines. For example, in one embodiment, apolyboronic acid may be reacted with itself. For instance, as shown inFIG. 19, 1,3,5-triboronic acid may undergo anhydride formation to formthe structure illustrated. In this embodiment, porous networks can beformed having a relatively small pore size. For instance, the porediameter of the structure shown in FIG. 19 may be about 6 angstroms.

In addition to selecting particular reactants in order to modify andchange the properties of the resulting porous network, in otherembodiments, functional groups may also be incorporated into theboronate linked network. For example, functional groups may beincorporated into one or both reactants that are used to form theboronate linked network. The functional groups can be incorporated intothe reactants by simple hydrogen substitution. Once incorporated intothe resulting boronate linked network, the functional groups may alterthe pore size of the resulting network and/or the ability of thematerial to adsorb gases or other organic constituents. Incorporatingfunctional groups into the porous network may also adjust thehydrophobic or hydrophilic properties of the resulting structure. Ofparticular advantage, it is believed that most functional groups willnot significantly alter the geometry around the borole being formed andwill only have minimal impact upon ester formation. In some embodiments,the functional groups will reduce pore volume and surface area whilegenerally increasing hydrophobicity. In some embodiments, functionalgroups may in fact enhance the stability of the structure. Thefunctional groups, for instance, may protect the boronate linkages fromhydrolytic degradation.

In general, any suitable functional group may be incorporated into theboronate linked network. The functional group may comprise, forinstance, a carboxy group, such as an acid, an ester, or an amide, ahalogen, an ether or any suitable hydrophobic group, such as an alkyl ora silyl, for example (tetramethylsilyl—TMS). It should be understood,however, that the above list is non-exhaustive and that any suitableorganic functional group may be used.

In one embodiment, for instance, the boronate linked networks may becarboxy-functionalized. Incorporating carboxy groups into the porousnetwork may bind polar species to the structure, such as volatileamines.

For exemplary purposes only, FIG. 16 illustrates one reaction schemethat may be used to incorporate functional groups onto a bis-diol. Asshown, 1,2,4,5-tetrahydroxybenzene may be functionalized at the 3- and6-positions with any suitable functional group, such as those listed inthe figure.

As shown in FIG. 16, in one embodiment, the polydiol may first bereacted with two methoxypropene in the presence of an acid catalyst inorder to initially protect the diol. In particular, acetal groups areformed. Next, deprotonation occurs using, for instance, n-butyl lithium.Functionalization results from reaction of the anion, generated byn-butyl lithium, with, for instance, an alkyl halide or carbon dioxide.The resulting carboxylic acid can be protected as the t-butyl orphenacyl ester or as an o-nitroanilide. The diol protecting group isremoved with acid. After the network is formed, the ester isdeprotected. Alternatively, the acid-functionalized starting materialcan be converted to an amide or ester. After the modification is made,the diol protecting group can be removed with an acid. Heating thet-butyl protected framework will release isobutene leaving behind thefree acid. Alternatively, the phenacyl esters and o-nitroanilidesprotecting groups are photo-labile.

As described above, incorporating functional groups into the reactantsmay diminish the resulting pore volume of the porous framework. Ifdesired, when incorporating functional groups into the boronate linkednetworks, longer reactants may be used to produce a proper pore sizewithout having the pores completely clogged by the functional groups.

As described above, incorporating functional groups into one of thereactants can be done through hydrogen substitution. The following is anon-exhaustive list of reactant species that may be used to produce aboronate linked network in accordance with the present disclosure thatinclude a functional group:

In the above formulas, R₁, R₂, R₃ and R₄ are independently hydrogen orany suitable functional group such as those described above. In certainembodiments, the same functional group may be incorporated into areactant at multiple locations. In other embodiments, only a singlefunctional group may be incorporated into the reactant. In still anotherembodiment, a reactant species may be formed containing multiplefunctional groups. For instance, a bis-diol may be formed as shown abovewherein R₁ is an alkyl group and R₂ is hydrogen, wherein R₁ is TMS whileR₂ is hydrogen, wherein R₁ is a carboxy group while R₂ is hydrogen,wherein R₁ is an ether while R₂ is a hydrogen. In other embodiments, R₁and R₂ may both comprise functional groups that can be the same ordifferent. For instance, in alternative embodiments, R₁ and R₂ aredifferent alkyl groups, R₁ is an alkyl group and R₂ is a silyl group, orR₁ is an alkyl group and R₂ is a carboxy group. It should be understoodthat all permutations are possible.

In the above examples, only polydiols and a polyboronic acid areillustrated. It should be understood, however, that similar functionalgroups may also be incorporated into polydiamines and into polyaminoalcohols.

As described above, porous networks can be formed according to thepresent disclosure having different characteristics and properties. Ingeneral, the resulting networks are highly crystalline. Pore diameterscan vary depending upon many factors. In general, the diameter of thepores within the porous network can be less than about 200 angstroms,such as less than about 100 angstroms, such as less than about 50angstroms. In one embodiment, for instance, porous networks can beformed according to the present disclosure having a pore diameter ofless than about 20 angstroms.

When formed into porous sheets, the spacing between adjacent sheets canalso vary. In one embodiment, the space between adjacent sheets is lessthan about 10 angstroms, such as from about 3 angstroms to about 5angstroms.

Of particular advantage, porous networks made according to the presentdisclosure are thermally stable. For instance, porous networks can beformed

that are thermally stable at temperatures of at least 400° C., such asat least 500° C.

Porous materials can be formed from the boronate linked networks thatare particularly well suited for gas adsorption. Of particularadvantage, gas adsorption is also reversible. The pore structures canhave a surface area of greater than at least about 300 m²/g, such asgreater than about 1000 m²/g. For instance, the surface area of thematerials can be from about 300 m²/g to about 2000 m²/g. Microporevolume can be from about 0.1 cc/g to about 0.5 cc/g, such as from about0.25 cc/g to about 0.35 cc/g. The above numerical ranges, however, areonly exemplary for various embodiments.

Porous networks made according to the present disclosure have numerouscommercial applications. For instance, since the porous network can beeasily changed and varied, the resulting product can be tailored to aparticular need. The ability to readily control the structure andfunction of the boronate linked networks allows them to be used innano-scale materials for potential applications in the areas ofseparations, catalysis, optical materials, and molecular sorption insensing. The porous networks, for instance, may find applications in gasstorage, catalysis, and separations.

The present disclosure may be better understood with reference to thefollowing example.

EXAMPLE

The following embodiment is provided to illustrate the present inventionand is not intended to limit the scope of the invention.

FIG. 1 shows the synthesis of the key building block to the formation ofthe covalent porous network shown in FIG. 4, benzene-1,3,5-triboronicacid. First, 1,3,5-Tris(trimethylsilyl)benzene was synthesized bycombining magnesium (7.77 g, 0.32 mol), dry THF (50 mL), andchlorotrimethylsilane (36.0 mL, 0.28 mol) in a round-bottom flask fittedwith a reflux condenser that had previously been charged with argon. Thesuspension was heated to 50° C. A solution of tribromobenzene (25.0 g,79.4 mmol) in dry THF (150 mL) was added drop-wise over a period ofabout 1 hour, and the reaction mixture was heated to reflux overnight.Consumption of the starting material was confirmed by TLC. The reactionmixture was cooled down to room temperature, passed though 2 inches ofsilica gel in a 3 inch diameter fritted funnel with hexanes, andsolvents were removed under reduced pressure. The crude oil was purifiedby column chromatography, eluting with hexanes, to yield the pure silane(11.00 g, 37.4 mmol), a clear and colorless oil.

Next, benzene-1,3,5-triboronic acid was formed by treating the silane(9.82 g, 33.4 mmol) with neat boron tribromide (41.0 g, 0.164 mol) underargon. A condenser was attached that was also charged with argon, andthe solution was heated at 100° C. for about 4 hours. Once cooled down,excess boron tribromide was distilled off under vacuum (1 Torr) at roomtemperature. The resulting grey-purple solid was dissolved in dry hexane(50 mL) and cooled down to 0° C. with an ice-bath. Water was slowlyadded drop-wise while stirring vigorously until the reaction had beenfully quenched. The grey solid was collected by filtration and rinsedwith water. After vacuum drying (1 torr) for 24 hours, the product wasobtained in near quantitative yield. The boronic acid functional groupswere capped with neopentaglycol in order to obtain furthercharacterization data on this monomer.

FIG. 2 shows the derivatization of benzene-1,3,5-triborinic acid to thecorresponding neopentaglycol triester. To synthesize the neopentaglycolester of benzene-1,3,5-triborinic acid, boronic acid (0.34 mmol) andneopentaglycol (116 mg, 1.11 mmol) were dissolved in methanol (2 mL).Methanol was removed under reduced pressure yielding the boronate linkednetworks quantitatively. The resulting white solid was recrystallizedfrom acetonitrile, giving colorless needles. The single crystal x-raystructure is shown in FIG. 2.

FIG. 4 shows an example of the covalent porous network (CPN-1) that wasreadily obtained through the condensation reaction betweenbenzene-1,3,5-triboronic acid 1 and 1,2,4,5-tetrahydroxybenzene 2 in 2%methanol/THF by refluxing under a constant flow of nitrogen for about 72hours. After cooling to room temperature, the resulting fine solid wascollected by filtration, washed with copious amounts of THF, and driedunder vacuum (1 Torr) at about 70° C. for about 72 hours to afford 96%yield of CPN-1 as a fine white powder.

CPN-1 was characterized using FTIR, ¹¹B and ¹H NMR to confirm that theexpected building blocks were indeed present in the assembly and thatthe desired bonds were formed. Infrared analysis of this material, asshown in FIG. 7, shows a significant attenuation for the hydroxylstretch compared to the starting materials indicating that a dehydrationreaction did in fact proceed. Furthermore, two intense peaks around 1300and 1345 cm⁻¹ signify that the boronate linked network functionality ispresent while peaks indicative of boron anhydride (boroxine) formation,namely a broad, intense peak at 1350 and a sharp peak at 580 cm⁻¹, wereabsent. Solid-state ¹¹B NMR analysis confirmed the presence of trigonalplanar boron centers revealing a single peak at 30 ppm compared to thestarting boronic acid monomer which appears at 25 ppm. To verify thatboth monomers were incorporated into the network, ¹H NMR analysis wasperformed using 1 M KOH in D₂0 as the solvent. Under these basicconditions the boronate linked material is hydrolyzed, providing asimple quantitative method to measure the composition in solution.Comparing the spectrum of degraded CPN-1 with starting materialsconfirms the presence of both monomers in the network. Integrationverifies that the bis-diol and tri-boronic acid are present in theexpected 3:2 ratio.

Powder X-ray diffraction (PXRD) analysis shows the formation of a highlycrystalline network. In order to evaluate the structure of the networkformed, the possible assemblies were modeled using Macromodel. Theproposed network contains pores with a diameter of approximately 18 Å.In conjunction with crystallographic data for analogous model compounds,Diamond software was used to define a unit cell with the origin locatedin the center of the pore. Based on these lattice parameters, expectedpowder diffraction patterns were generated assuming that the tri-boronicacid and bis-diol building blocks assembled as proposed. Planartwo-dimensional sheets were expected and the manner in which thesesheets can stack is limited. The most likely crystal packing allows thesheets to assemble in a registered AA manner where atoms in adjacentsheets lie directly over each other forming large cavities and leaving ahexagonal array of 1D, 18 Å pores (FIG. 11); alternatively they couldassemble in a staggered AB arrangement, where the sheets offset by ½, ½placing the phenyl rings in every other layer in registry whilepartially blocking the pore of the intermediate layer (FIG. 12).Although both models have primitive hexagonal symmetry, they woulddisplay distinct diffraction patterns (FIG. 13).

There are too few peaks observed in the PXRD of CPN-1 to use theRietveld method to refine atomic positions; however, the LeBail routinecan be used (FIG. 14) to compare calculated and experimental PXRDpatterns in order to determine the most probable structural motif. InFIG. 13, the experimental powder diffraction pattern is compared withthe calculated patterns generated using the atomic positions determinedfrom Diamond. There is substantial correlation between the expected peakposition and intensities for the registered AA model and theexperimental data, whereas the projected pattern for the staggered ABarrangement does not fit experimental data. Consistent with the modeledstructure, the PXRD supports the presence of 18 Å micropores, arrangedin a hexagonal orientation. The interlayer spacing between sheets frommodeling versus that found from the PXRD analysis agree well (3.597 Åcalc. versus 3.385 Å exp.).

The porosity and pore stability of CPN-1 were evaluated by measuring theN₂ gas adsorption. This material exhibits thermal stability totemperatures up to 500° C., as shown by thermogravimetric analysis (FIG.8). As such, a sample of CPN-1 was evacuated under dynamic vacuum (10⁻⁵Torr) while heated at 400° C. for about 2.5 hours. This sample was usedfor gas adsorption measurements from 0 to 760 Torr at 77 K. Theadsorption profile was reversible and reproducible and showed a verysharp uptake at low partial pressure (P/Po from 4×10⁻⁶ to 0.01) which isindicative of microporous material (FIG. 9). Using the BET model (FIG.10), the apparent surface area was determined to be about 1350 m²/g,which corresponds to a micropore volume of about 0.29 cm³/g (FIG. 10).

In summary, the present invention is a novel method of synthesizing andcharacterizing a microporous, covalently-linked, poly(boronate) networkwith persistent pores. Spectral characterization has confirmed thebonding motif to generate infinite 2D porous sheets while PXRD was usedto define the long range ordering of these sheets, such that atoms inadjacent layers lie directly over each other resulting in a hexagonalarray of 1D, 18 nm pores. The resulting Covalent Porous Network (CPN) isthermally stable to 500° C. and maintains a higher surface area thanmost known porous materials. Given the enhanced stability, high surfacearea and small micropore volume, this CPN appears ideally suited toserve as a matrix for gas adsorption.

These and other modifications and variations to the present inventionmay be practiced by those of ordinary skill in the art, withoutdeparting from the spirit and scope of the present invention, which ismore particularly set forth in the appended claims. In addition, itshould be understood that aspects of the various embodiments may beinterchanged both in whole or in part. Furthermore, those of ordinaryskill in the art will appreciate that the foregoing description is byway of example only, and is not intended to limit the invention sofurther described in such appended claims.

What is claimed:
 1. A process for forming a boroxine linked porousnetwork comprising: reacting a polyboronic acid or an acyclic boronateester thereof with itself to form an anhydride porous network.
 2. Theprocess as defined in claim 1, the porous network defining pores havinga pore diameter of less than about 200 angstroms.
 3. The process asdefined in claim 1, wherein the polyboronic acid or an acyclic boronateester thereof comprises a triboronic acid or an acyclic boronate esterthereof.
 4. The process as defined in claim 3, wherein the triboronicacid comprises benzene-1,3,5-triboronic acid or an acyclic boronateester thereof.
 5. The process as defined in claim 1, wherein thepolyboronic acid or an acyclic boronate ester thereof comprises atriboronic acid.
 6. The process as defined in claim 1, wherein thepolyboronic acid or an acyclic boronate ester thereof comprises:


7. The process as defined in claim 1, wherein the boroxine has thefollowing structure:


8. The process as defined in claim 1, wherein the polyboronic acid or anacyclic boronate ester thereof comprises:

or an acyclic boronate ester thereof.